docs: def get_file_size(filename):
      """
      Get size of all files in gigabytes (Gb).
  
      :param str | collections.Iterable[str] filename: A space-separated
          string or list of space-separated strings of absolute file paths.
      :return float: size of file(s), in gigabytes.
      """
      if filename is None:
          return float(0)
      if type(filename) is list:
          return float(sum([get_file_size(x) for x in filename]))
      try:
          total_bytes = sum([float(os.stat(f).st_size)
                             for f in filename.split(" ") if f is not ''])
      except OSError:
          # File not found
          return 0.0
      else:
          return float(total_bytes) / (1024 ** 3)